Controlling Light In Photography: Working With Apertures And Shutter Speeds

Photography is defined as "the art and science of altering images on a sensitive surface through the action of light", thus providing proof that understanding of the nature of light, and being able to control it, is fundamental to success as a photographer.  The focus of the information below will be on the behavior of light and how these particular behaviors affect portraiture photography.

In portrait photography there are many ways that light can affect the image being captured, which can either detract or add to its artistic success.  One of the fundamental aspects of light is exposure. Whether using film or digital, the photosensitivity on the surface to which an image is affixed is what ultimately dictates the intensity and length of exposure of said surface to light.

Working With The Right Aperture

The aperture of the modern camera lens is what allows the photographer to control the amount of light that falls on the film or the camera's image sensor. Basically, the aperture is just an opening that allows light to enter the camera from the subject, and the intensity of this light that is used to expose the film or image sensor is controlled mainly by the size of this hole.

The size of the aperture opening is commonly measured in f/stops.  The f/stop is number that represents a fractional opening of the aperture in the lens. A decrease in one f/stop will cause the amount of light allowed into the camera to double.  Alternately, an increase in one f/stop will reduces the amount of light admitted into the camera by half.

Proper exposure is not just dictated by the amount of light that seems suitable for the image being captured, but also depends on the depth of field as well.  The depth of field is defined as the distance in front of and behind the subject, which is in focus.  To simplify this concept, it is agreed that, generally, as the depth of field increases, the size of the aperture opening decreases and vice versa.

Understanding The Shutter Speed Concept

Adjusting the duration of the light hitting the film or image sensor is also pivotal to the success of a photograph.  This adjustment is controlled by the shutter, which is effective in controlling the length of the exposure. Simplistically, the shutter is a type of metal curtain with a slit in it that passes in front of the film or image sensor at a pre-selected speed.

Shutter speeds can be measured in minutes, seconds or fractional seconds.  For example, a shutter speed of 1/100th of a second lets double the duration of exposure as a shutter speed of 1/200th of a second.  The resolution of an image is partially determined by the length of exposure in taking the photograph.  

A standard rule of portrait photography is to set the aperture size to give the appropriate depth of field, and then set the shutter speed to obtain the correct exposure level.

ISO Speed (For Film Users)

Another fundamental parameter that can be adjusted to control exposure of an image is the film speed or ISO/ASA number. Film speed is a quantitative description of the chemical based photosensitivity of any material used in the film.

The higher the ISO or ASA number, the more photosensitive the film or sensor is. For example, faster film speeds are appropriate for quick moving subjects or low-lit subjects as they allow images of such subjects to be more easily captured.

The drawback of faster film speeds is the common presentation of graininess or "noise" in an image, coupled with loss of detail and sharpness. Similar to the increase in film speed, the ISO or ASA number on most modern digital cameras can be adjusted to manually control the digital image sensor's sensitivity, which also increases the "noise" in an image.

The important message being conveyed can be summed up as the following: all light is controlled inside the camera by setting the duration and intensity of exposure to a certain level, and by choosing an appropriate film speed for the light conditions, or adjusting the sensitivity of the digital image sensor.

For a finer degree of controlling light in photography, usually used as an effective tool to enhance specific highlights, falloff, and shadow softness, can be controlled outside the parameters of the camera itself.

Homework

Take a lens that allows you to go as down as 1.8 on its aperture scale, and see how you need a faster shutter speed to compensate for the light. Then simply travel on the aperture scale up till F22. When your shutter speed cannot compensate, change ISO settings (if on a digital camera). See how you have to balance each three to obtain different effects.